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Steps of Injection Mold Design
Szimonetta Szekér
The Most Important Steps
- Product and Machine Analysis – Review of drawing tolerances and parameters
- Manufacturability Analysis - Examination of plastic type, shrinkage, draft angles, and undercuts; making recommendations and creating pre-simulations if necessary
- Parting Line Definition, Surface Modeling Defining opening direction(s), creating the parting surface
- Insert Design Creating inserts considering manufacturability and venting, designing the runner system
- Cooling System Design Designing the cooling circuits with ejection in mind
- Ejection System Design Determining and modeling the position of ejector pins, designing the ejection system
- Mold Base Design Structuring the mold plates, guide elements, and external mold components
- BOM, Drawing Creation, Production Preparation Material selection, defining manufacturing parameters, creating 2D drawings, issuing technological instructions, designing fixtures and electrodes
1. Product and Machine Analysis
The first step in mold design is the analysis of the product and the manufacturing environment based on customer documentation. It is necessary to examine the 2D drawings and specifications, and to check the dimensional tolerances, surface requirements, and any special needs (e.g., visible surfaces, sealing surfaces, etc.). It is equally important to know the parameters of the injection molding machine: which machine the mold will be mounted on, the maximum mold size, maximum opening stroke, minimum mold size, clamping method, the size of the locating ring required, what electrical and water connections are available, and the method of ejection. I also include the analysis of the raw material in this first step; the correct determination of shrinkage is one of the most crucial points in mold design. An incorrectly chosen shrinkage value can lead to significant extra costs and time during the optimization process.
2. Manufacturability Analysis
An injection-moldable product must meet certain geometric and technological conditions. This includes ensuring an appropriate wall thickness for uniform material flow – for example, excessively thick walls can cause sink marks and voids, while walls that are too thin can lead to filling problems. The use of draft angles is also essential. This is also where we examine undercuts.
3. Parting Line Definition, Surface Modeling
The next important step is to define the parting line. The parting line, or parting surface, is the boundary surface where the mold will open. It is where the stationary and moving mold halves meet. The internal surfaces are placed on the moving side, as the part shrinks onto this side, while the external surfaces are placed in the stationary side. In the case of undercut parts, the product cannot be removed in the opening direction, so side movements and moving elements must be used for these areas, such as sliders, lifters, collapsing cores, or flying inserts.
4. Insert Design
After modeling the parting surface, the next step is to define other inserts. Here, venting must be considered; it may be necessary to place inserts in critical areas (e.g., near ribs) to allow air to escape. Manufacturability is an important aspect, as is the consideration of critical dimensions; it is also worth using inserts in such positions so that dimensions with tighter tolerances can be more easily corrected during the optimization process.
I also include the design of the runner system in this point, during which the hot, cold, or hybrid runner specified by the customer is designed. If a hot runner system is required, it is always designed by its manufacturer; the mold designer must ensure its integrability and the provision of appropriate drawing tolerances.
In the case of a cold or mixed runner, the selection of the runner's cross-section is a key element, both in terms of its size and shape, which can be circular, trapezoidal, or semi-circular. Its size depends on the volume of the product. In the case of multiple cavities, it must be designed so that all cavities begin to fill at the same time, and attention must also be paid to the placement of cold slug wells, which serve to ensure that the initial, "cooled" portion of the material flows there and not into the product. The type of gate is also determined at this time, which depends on the customer's specifications. Some customers may specify where it should be located and what type it should be. For example, a tunnel or banana gate (a self-degating type) is needed if the expectation is that a finished part will fall out of the mold without any post-processing. Whereas, if we use a film gate or a direct gate, the runner must be removed from the product.
5. Cooling System Design
When designing the cooling system, the product's raw material and geometry must be taken into account. The circuits must be integrated to follow the shape of the part as closely as possible, with the most uniform flow. A well-designed cooling system can reduce warpage. The medium is usually water, but for parts molded at high temperatures, it can also be oil. With proper cooling, the cycle time can also be optimized.
6. Ejection System Design
When placing the ejectors, the following aspects are critical: The product must be removable from the mold, and a suitable ejection stroke must be chosen so that the part emerges from the cavity. In the case of robotic removal, a shorter ejection stroke is sufficient. For critical features, it is important to place an ejector for both venting and ejection purposes, for example, for deep ribs. It is also important to ensure that there is no collision with other mold components.7. Mold Base Design
When determining the dimensions of the mold base, the dimensions of the injection molding machine's platen must be taken into account. It is advisable to use standard plate sizes from a Meusburger or Hasco catalog. This is when the guide pillars, other guide elements, locating rings, and support plates are incorporated.
The parts of the ejection system are also installed, such as the return pins (which serve for the mechanical protection of the system), the support pillars (which are necessary for support), and the guidance for the ejection system.In addition to mechanical protection, electronic protection is also found here, in the form of a limit switch. Buffer plates are needed above and below the ejector plates. This ensures that the system does not bottom out on its full surface at either end of its stroke.
As the final step of modeling, the external mold components are added to the mold: the lifting and eyebolts, which are necessary for transport; the mold transport lock, which is important for protecting the mold so that it does not open during lifting and strain the guide elements. A plexiglass plate is also needed in front of the ejection system to prevent anything from falling into or reaching into the ejection system.
8. BOM, Drawing Creation, Production Preparation
As the final step, we create the BOM (Bill of Materials) list. This is when the selection of raw materials takes place, which depends on the expected lifetime of the mold and the type of plastic material. At this time, we determine the sourcing for standard components and other raw material or insert specifications. We create 2D drawings for every element to be manufactured or modified, indicating the necessary tolerances and specifications. Taking all of this into account, the sequence of technological instructions is determined, and fixtures and electrodes are designed.
Summary
The design of an injection mold is a complex and responsible process in which the smallest details determine the quality of the final product and the efficiency of production. The key to success is proper preparation: thorough analysis of the product and the manufacturing environment, ensuring manufacturability, precise parting line and insert design, and a well-designed cooling and ejection system. Equally important is the proper selection of the mold base and standard components, as well as accurate documentation prepared during production preparation.
A well-thought-out mold guarantees not only the quality of the product but also the cost-effectiveness of production, the cycle time, and long-term operational reliability.
This article is not exhaustive but rather provides an insight into the most important steps and aspects of mold design. The details can vary significantly by industry, product, and customer expectations, so the actual design process requires much deeper professional knowledge and experience.
